ABSTRACT: This study investigated the effect of UV-A and B (UVR) on larval settlement of the polychaete Hydroides elegans through their influence on biofilms. Multispecies biofilms were treated with 3 doses of UVR (10, 30 and 80 KJ m2) at 2 environmentally realistic irradiance levels (4 and 2 W m2) in the laboratory, and their ability to induce the settlement of H. elegans larvae was then examined in both laboratory and field conditions. For the field experiment, only 10 and 80 KJ m2 doses under 4 W m2 were used. In addition, this study evaluated the effects of UVR on monospecies bacterial biofilms and then their ability to induce larval settlement in the laboratory. Results demonstrated that the ability of multispecies biofilms to trigger larval settlement could be compromised as a result of enhanced UVR exposure. Although larval settlement on multispecies biofilms treated with the lowest UV dose (at both irradiance levels) was at the same level as that of the control, the exposure of biofilms to the highest dose significantly reduced their larval settlement triggering ability. Furthermore, UVR treatments decreased the percentage of metabolically active bacterial cells in monospecies biofilms; the effect increased with increasing UV dose. Larval settlement response to monospecies biofilms decreased with increasing UV dose, suggesting that the bacterial metabolic activity in biofilms is essential for the biofilms to have an inductive effect on larval settlement. This study suggests that enhanced UVR, which might occur due to ozone depletion, may have a significant effect on the larval settlement of H. elegans by affecting a biofilms inductive cues.